1. Field of the Invention
The present invention relates to a control unit for two variable displacement compressors used for two independent air-conditioning systems.
2. Description of the Related Art
In the prior art, a variable displacement compressor of a regularly operative type for an air-conditioner of a vehicle, which is regularly operative by a power from an engine provided the engine is driven, is known from Japanese Unexamined Patent Publication (Kokai) No. 2000-220577. This compressor is capable of optionally changing the operating displacement in a range of approximately 0% to 100% by electric signals from an electronic control unit (ECU) in accordance with various operating conditions. When the air conditioning in a passenger compartment is necessary, the compressor is switched to an ON mode in which a coolant is compressed by the compressor based on the signal from ECU and discharged to an air-conditioning cycle. If the air conditioning in the passenger compartment is unnecessary, the compressor is switched to an OFF mode, which is a minimum displacement operation, and no coolant is discharged to the air conditioning cycle.
While sliding portions in the interior of the compressor usually become hot due to self-heating during the operation, in the case of the ON mode, the sliding portions are cooled because coolant at a low temperature is sucked from the air conditioning cycle into the compressor. On the other hand, in the case of the OFF mode, in which no air conditioning is carried out, as no coolant is sucked from the air conditioning cycle into the compressor, the interior sliding portions of the compressor are not cooled by the coolant. When the air is at a relatively low temperature, the compressor is usually driven in the OFF mode as the passenger requires no air conditioning. However, the interior temperature of the compressor does not rise to a dangerous value because the outer air temperature is low. Conversely, in an environment with a high air temperature, the compressor is driven in the ON mode because the air conditioning is usually necessary, and the compressor temperature does not rise to an extraordinary high value.
However, in a case of a large-sized vehicle such as a limousine having two air conditioning systems using two compressors, completely independent from each other, in which one compressor is used for the front seats and another compressor is used for the rear seats, the compressor for the rear seats may be operated in the OFF mode when the air temperature is high but there are no passengers in the rear seats. When the compressor is driven in the OFF mode at such a high air temperature, no cooling effect is obtainable from a returning coolant from the air conditioning cycle. Thus, the temperature of the compressor reaches a dangerous temperature due to self-heating to result in the seizing of the compressor in the worst case.